This invention relates to a method and apparatus for coating an elongated filament. More particularly, it relates to a method and apparatus for applying to an optical waveguide fiber a concentric coating having a low incidence of bubbles.
Glass optical waveguide fibers must exhibit high strength in order to withstand the stresses which are encountered in incorporating them into protective sheathing or cable, installing the cable, or in use. While such fibers are typically quite strong as drawn from a preform crucible or the like, this strength is rapidly degraded by surface defects which are introduced into the fiber through handling or otherwise.
To preserve the strength of a newly drawn fiber, it is conventional to apply to the fiber immediately after it is drawn a thin protective coating composed of an organic or inorganic coating material to protect it from surface abrasion during subsequent handling.
Tapered dies of the type employed for the application of coatings to conductive wires cannot be used for coating glass optical fibers. In one such die, the wire is maintained in a central position within the extrusion die by a tip orifice which is just large enough to permit the conductive wire to pass therethrough. Although such an apparatus is suitable for centering a conductive wire within a coating die, it would obviously be unsuitable for coating glass optical fibers which must not touch any solid surface prior to or during the coating step.
Apparatus for coating glass optical fibers typically comprises a reservoir containing coating fluid, a small, exit orifice being situated at the bottom thereof. The reservoir may consist of a tapered die having means at the top thereof or along the sidewall thereof for introducing coating liquid under pressure. The reservoir can also have a cylindrical or other suitable shape in which case the bottom surface thereof is usually provided with a tapered die. The fiber enters the coating liquid at the surface thereof and exits through the die orifice. Downward movement of the fiber through the free surface of the coating fluid generates a downwardly extending meniscus at the surface. At relatively low flow rates the meniscus is properly positioned, in part by the shape of the reservoir, thus centering the fiber within the coating.
Fibers can be more economically produced by increasing the draw rate. However, as speeds exceed 3 m/s, secondary flows within the coating liquid become intensified, and the fiber begins to draw air into the coating fluid. The fiber is thus incompletely wet by the fluid, and air bubbles accumulate in the coating liquid. Bubbles which get entrapped in the secondary flow motion within the liquid cannot escape to the top surface thereof. As bubble density increases, some bubbles pass through the die and remain in the coating. Bubble generation and secondary flows inside the applicator reduce the efficiency of fluid centering forces inside the applicator. The result can be poor coating concentricities and poor coating characteristics such as coating separation and incomplete curing.
In accordance with the teachings of U.S. Pat. No. 4,374,161 the fiber is pulled through a coating chamber passage that extends axially through a feed cylinder. Coating liquid is directed under pressure radially inwardly toward the fiber. The coating liquid is subjected to high pressure for the purpose of preventing air from entering the passage as the fiber is pulled therethrough. A passage diameter of from 1.5 to 5 times the fiber diameter is said to be sufficiently large to prevent contact of the fiber with the sides of the passage. In one specific example described in that patent, the cylinder contains many small radially-extending rectangular holes, there being four circumferentially spaced holes per 0.03 inch thick section, each hole having a cross-sectional area of 0.00004 in.sup.2 (0.00026 cm.sup.2). Extremely high pressures are required for delivering an adequate amount of coating liquid to the fiber drawn at 1 m/sec, said example specifying a pressure of 200 psi at a coating temperature of 160.degree. C. Furthermore, in order to supply the fiber with a sufficient amount of liquid, a cylinder length of 1.5 inch (3.8 cm) was required. Draw speeds as low as 3 m/sec would require an inordinately high pressure. Also, any instability caused by such higher draw speeds increases the probability of the fiber contacting the surface of the small diameter coating chamber.